Dienes, especially isoprene, are useful as monomers for the manufacture of synthetic rubbers. Several fundamental processes have been used to construct the isoprene C.sub.5 skeleton from smaller carbon units. These processes are not commercially accepted in that there are numerous problems associated with each particular synthesis route.
One avenue to the construction of a C.sub.5 skeleton uses linear butenes which are catalytically isomerized to a mixture of cis- and trans-butene-2 and then hydroformylated to produce 2-methylbutanal (2MBA) in the presence of a homogeneous rhodium catalyst and an organic ligand. The 2MBA is then dehydrated to isoprene in the presence of acidic heterogeneous catalysts at elevated temperatures. U.S. Pat. No. 4,524,233 discloses and claims a process for the conversion of an aldehyde to a diolefin comprising contacting an aldehyde of 4 to 6 carbon atoms in the vapor phase at a temperature of from 200.degree. to 400.degree. C. with a boron phosphate catalyst wherein the initial molar ratio of phosphorus to boron is less than 1 but greater than 0.6 which is in intimate admixture with from 0.1 to 10 weight percent graphite. This patent discloses the dehydration of aldehydes such as 2-methylbutanal, 2,3-dimethylbutanal and 2- or 3-ethylbutanal. The dienes that are produced as a result of this process include 1,3-butadiene, isoprene, 1,3-hexadiene, 2, 3, or 4-methyl-1,3-pentadiene, 2,3-methylbutadiene and 2-ethyl-1,3-butadiene.
Those knowledgeable in the art of polymer synthesis are constantly searching for new monomers which may be useful in the preparation of polymers that possess unique physical properties. Two monomers of particular interest are 3-methyl-1,3-pentadiene and 2-ethyl-1,3-butadiene. These monomers can be prepared from the dehydration of 2-ethylbutanal. In the dehydration of 2-ethylbutanal, the major dehydration products produced are ethyl butadiene and 3-methyl-1,3-pentadiene with the production of 3-methyl-1,3-pentadiene being predominate. It would be especially useful to have a process which would allow for the conversion of 2-ethyl-1,3-butadiene to 3-methyl-1,3-pentadiene or vice versa.
An article by Higashimura and Hasegawa, Journal of Polymer Science, Vol. 17, 2439-2446 (1979) entitled, "Monomer-Isomerization Oligomerization of 2-Ethyl-1,3-Butadiene by Acid Catalysts" discloses the cationic oligomerization of 2-ethyl-1,3-butadiene (2EBD) by a superacid which is accompanied by monomer isomerization to 3-methyl-1,3-pentadiene (3MPD) before propagation to yield oligomers of the isomerized monomer as the main product in benzene at 50.degree. C. This article deals with the oligomerization of 2-ethyl-1,3-butadiene which is easier to handle at high temperatures than volatile butadiene and isoprene. The authors found that a superacid catalyzed oligomerization of 2EBD initially isomerizes to 3-methyl-1,3-pentadiene before propagation and yields primarily 3MPD oligomers. This article does not suggest or disclose the instant discovery that a specific boron phosphate catalyst will allow for the conversion of these branched dienes without oligomerization or polymerization of the desired monomeric material.
U.S. Pat. No. 3,607,968 is concerned with the isomerization of aliphatic olefins having 9 to 22 carbon atoms in the presence of a boron phosphate catalyst at elevated temperatures. This patent does not suggest or disclose the use of a specific boron phosphate catalyst under specific conditions to achieve the desired isomerization of a branched diene.
U.S. Pat. No. 4,537,995 is concerned with a process for isomerizing branched aldehydes to ketones which comprises contacting at isomerization conditions a branched aldehyde with a zeolite catalyst. The crystalline aluminosilicate zeolites used in this patent are referred to generally as ZSM-5 type or behaving like ZSM-5 type zeolites. The process is typically conducted in the vapor phase utilizing temperatures as low as 325.degree. C.
U.S. Pat. No. 4,632,913 is concerned with an improved boron phosphate dehydration catalyst wherein the boron phosphate has been treated with an ammonium carbonate or bicarbonate salt prior to calcination.
U.S. Pat. No. 4,628,140 is directed to an improved process for the conversion of an aldehyde to a diolefin comprising contacting an aldehyde of 4 to 6 carbon atoms in the vapor phase with a catalyst, the improvement comprising the addition of from 0.10 to 5 percent by weight of an aromatic compound to the aldehyde feed.
The use of acid catalysts to perform isomerization reactions is known and boron phosphate is known to catalyze certain isomerization reactions: however, there is no suggestion or disclosure from the prior art that a specific boron phosphate catalyst wherein the P/B ratio is less than 1 would allow for the conversion of 3-ethyl-1,3-butadiene to 3-methyl-1,3-pentadiene and vice versa with exceptional conversions and high selectivities without the attendant problems of catalysts fouling, reduced catalyst lifetimes or diene oligomer formation.
A disadvantage associated with known catalysts to isomerize unsaturated materials is that catalyst life depends on many factors which includes catalyst composition, structure, catalyst activity, operating temperatures and coke deposition. Coke deposition is understood to denote coke (carbonaceous) deposits formed on the catalyst during the isomerization reaction. The use of boron phosphate as a catalyst for the isomerization of monoolefins and aldehydes is known: however, the problems associated with a diene isomerization are different and more difficult to overcome than those found in monoolefin or aldehyde isomerizations. The isomerization of a diolefin such as 2EBD or 3MPD is difficult since these highly reactive monomers are known to form dimers and/or polymerize in the presence of acid catalysts. Due to these and other differences, catalysts suitable for long term isomerization of dienes have not found acceptance in the preparation of these desirable monomeric materials.
One aspect of this invention is directed to the use of graphite which is an intimate physical admixture with the boron phosphate. Graphite has numerous known uses including utility as carrier for catalysts and as a lubricant during catalyst pellet formation. See U.S. Pat. No. 1,841,055. The prior art does not suggest, disclose or appreciate that the presence of graphite in a boron phosphate catalyst will unexpectedly enhance the viable lifetime of the catalyst in a diene isomerization reaction.
A portion of the instant invention is directed to a catalyst of high selectivity and low coke deposition in conjunction with extended catalyst life times. The prior art does not suggest or disclose a specific catalyst or process for the isomerization of dienes which would be suitable for commercial application.